Echo device method for locating upstream ingress noise gaps at cable television head ends

ABSTRACT

A system and method for locating ingress noise gaps for an upstream data carrier in a cable network utilizing cable modems is described. A system for identifying a transmission frequency that has less noise than other available frequency bands includes a packet generator, a rate controller, an echo device, a demodulator, and a packet checker. The packet generator and rate controller, contained in a cable modem termination system, send test data downstream to the echo device which then redirects the test data upstream at a selected test frequency. The echo device, which may be located within the cable modem termination system or outside the termination system, listens on the downstream path for data packets addressed to it. Once received, the echo changes the address of the test data to the address of the packet checker. The packet checker then receives and analyzes the test data packet sent from the echo device to assess the amount of noise on that particular test frequency. The echo device can be a subscriber cable data modem configured to receive test packets sent downstream or a custom-built device located anywhere on the downstream path or within the cable modem termination system.

This is a Continuation application of copending prior application Ser.No. 08/962,231 filed on Oct. 31, 1997, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field Of Invention

The present invention relates generally to methods and apparatus fortransmitting digital data in cable television network systems. Morespecifically, the present invention relates to methods and apparatus forlocating upstream ingress noise gaps for cable modems.

2. Background

The cable TV industry has been upgrading its signal distribution andtransmission infrastructure since the late 1980s. In many cabletelevision markets, the infrastructure and topology of cable systems nowinclude fiber optics as part of its signal transmission component. Thishas accelerated the pace at which the cable industry has taken advantageof the inherent two-way communication capability of cable systems. Thecable industry is now poised to develop reliable and efficient two-waytransmission of digital data over its cable lines at speeds orders ofmagnitude faster than those available through telephone lines, therebyallowing its subscribers to access digital data for uses ranging fromInternet access to cablecommuting.

Originally, cable TV lines were exclusively coaxial cable. The systemincluded a cable head end, i.e. a distribution hub, which receivedanalog signals for broadcast from various sources such as satellites,broadcast transmissions, or local TV studios. Coaxial cable from thehead end was connected to multiple distribution nodes, each of whichcould supply many houses or subscribers. From the distribution nodes,trunk lines (linear sections of coaxial cable) extended toward remotesites on the cable network. A typical trunk line is about 10 kilometers.Branching off of these trunk lines were distribution or feeder cables(40% of the system's cable footage) to specific neighborhoods, and dropcables (45% of the system's cable footage) to homes receiving cabletelevision. Amplifiers were provided to maintain signal strength atvarious locations along the trunk line. For example, broadbandamplifiers are required about every 2000 feet depending on the bandwidthof the system. The maximum number of amplifiers that can be placed in arun or cascade is limited by the build-up of noise and distortion. Thisconfiguration, known as tree and branch, is still present in oldersegments of the cable TV market.

With cable television, a TV analog signal received at the head end of aparticular cable system is broadcast to all subscribers on that cablesystem. The subscriber simply needed a television with an appropriatecable receptor to receive the cable television signal. The cable TVsignal was broadcast at a radio frequency range of about 60 to 700 MHz.Broadcast signals were sent downstream; that is, from the head end ofthe cable system across the distribution nodes, over the trunk line, tofeeder lines that led to the subscribers. However, the cable system didnot have installed the equipment necessary for sending signals fromsubscribers to the head end, known as return or upstream signaltransmission. Not surprisingly, nor were there provisions for digitalsignal transmission either downstream or upstream.

In the 1980s, cable companies began installing optical fibers betweenthe head end of the cable system and distribution nodes (discussed ingreater detail with respect to FIG. 1 below). The optical fibers reducednoise, improved speed and bandwidth, and reduced the need foramplification of signals along the cable lines. In many locations, cablecompanies installed optical fibers for both downstream and upstreamsignals. The resulting systems are known as hybrid fiber-coaxial (HFC)systems. Upstream signal transmission was made possible through the useof duplex or two-way filters. These filters allow signals of certainfrequencies to go in one direction and of other frequencies to go in theopposite direction. This new upstream data transmission capabilityallowed cable companies to use set-top cable boxes and allowedsubscribers pay-per-view functionality, i.e. a service allowingsubscribers to send a signal to the cable system indicating that theywant to see a certain program.

In addition, cable companies began installing fiber optic lines into thetrunk lines of the cable system in the late 1980s. A typical fiber optictrunk line can be upto 80 kilometers, whereas a typical coaxial trunkline is about 10 kilometers, as mentioned above. Prior to the 1990s,cable television systems were not intended to be general-purposecommunications mechanisms. Their primary purpose was transmitting avariety of entertainment television signals to subscribers. Thus, theyneeded to be one-way transmission paths from a central location, knownas the head end, to each subscriber's home, delivering essentially thesame signals to each subscriber. HFC systems run fiber deep into thecable TV network offering subscribers more neighborhood specificprogramming by segmenting an existing system into individual servingareas between 500 to 2,000 subscribers. Although networks usingexclusively fiber optics would be optimal, presently cable networksequipped with HFC configurations are capable of delivering a variety ofhigh bandwidth, interactive services to homes for significantly lowercosts than networks using only fiber optic cables.

FIG. 1 is a block diagram of a two-way hybrid fiber-coaxial (HFC) cablesystem utilizing a cable modem for data transmission. It shows a headend 102 (essentially a distribution hub) which can typically serviceabout 40,000 subscribers. Head end 102 contains a cable modemtermination system (CMTS) 104 that is needed when transmitting andreceiving data using cable modems. CMTS 104 is discussed in greaterdetail with respect to FIG. 2. Head end 102 is connected through pairsof fiber optic lines 106 (one line for each direction) to a series offiber nodes 108. Each head end can support normally up to 80 fibernodes. Pre-HFC cable systems used coaxial cables and conventionaldistribution nodes. Since a single coaxial cable was capable oftransmitting data in both directions, one coaxial cable ran between thehead end and each distribution node. In addition, because cable modemswere not used, the head end of pre-HFC cable systems did not contain aCMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by acoaxial cable 110 to two-way amplifiers or duplex filters 112 whichpermit certain frequencies to go in one direction and other frequenciesto go in the opposite direction (frequency ranges for upstream anddownstream paths are discussed below). Each fiber node 108 can normallyservice upto 500 subscribers. Fiber node 108, coaxial cable 110, two-wayamplifiers 112, plus distribution amplifiers 114 along trunk line 116,and subscriber taps, i.e. branch lines 118, make up the coaxialdistribution system of an HFC system. Subscriber tap 118 is connected toa cable modem 120. Cable modem 120 is, in turn, connected to asubscriber computer 122.

Recently, it has been contemplated that HFC cable systems could be usedfor two-way transmission of digital data. The data may be Internet data,digital audio, or digital video data, in MPEG format, for example, fromone or more external sources 100. Using two-way HFC cable systems fortransmitting digital data is attractive for a number of reasons. Mostnotably, they provide upto a thousand times faster transmission ofdigital data than is presently possible over telephone lines. However,in order for a two-way cable system to provide digital communications,subscribers must be equipped with cable modems, such as cable modem 120.With respect to Internet data, the public telephone network has beenused, for the most part, to access the Internet from remote locations.Through telephone lines, data is typically transmitted at speeds rangingfrom 2,400 to 33,600 bits per second (bps) using commercial (and widelyused) data modems for personal computers. Using a two-way HFC system asshown in FIG. 1 with cable modems, data may be transferred at speeds upto 10 million bps. Table 1 is a comparison of transmission times fortransmitting a 500 kilobyte image over the Internet.

                  TABLE 1                                                         ______________________________________                                        Time to Transmit a Single 500 kbyte Image                                     ______________________________________                                        Telephone Modem (28.8 kbps)                                                                      6-8        minutes                                         ISDN Line (64 kbps)                                                                                       1-1.5                                                                           minutes                                         Cable Modem (30 Mbps)                                                                                       second                                          ______________________________________                                    

Furthermore, subscribers can be fully connected twenty-four hours a dayto services without interfering with cable television service or phoneservice. The cable modem, an improvement of a conventional PC datamodem, provides this high speed connectivity and is, therefore,instrumental in transforming the cable system into a full serviceprovider of video, voice and data telecommunications services.

As mentioned above, the cable industry has been upgrading its coaxialcable systems to HFC systems that utilize fiber optics to connect headends to fiber nodes and, in some instances, to also use them in thetrunk lines of the coaxial distribution system. In way of background,optical fiber is constructed from thin strands of glass that carrysignals longer distances and faster than either coaxial cable or thetwisted pair copper wire used by telephone companies. Fiber optic linesallow signals to be carried much greater distances without the use ofamplifiers (item 114 of FIG. 1). Amplifiers decrease a cable system'schannel capacity, degrade the signal quality, and are susceptible tohigh maintenance costs. Thus, distribution systems that use fiber opticsneed fewer amplifiers to maintain better signal quality.

In cable systems, digital data is carried over radio frequency (RF)carrier signals. Cable modems are devices that convert digital data to amodulated RF signal and convert the RF signal back to digital form. Theconversion is done at two points: at the subscriber's home by a cablemodem and by a CMTS located at the head end. The CMTS converts thedigital data to a modulated RF signal which is carried over the fiberand coaxial lines to the subscriber premises. The cable modem thendemodulates the RF signal and feeds the digital data to a computer. Onthe return path, the operations are reversed. The digital data is fed tothe cable modem which converts it to a modulated RF signal (it ishelpful to keep in mind that the word "modem" is derived frommodulator/demodulator). Once the CMTS receives the RF signal, itdemodulates it and transmits the digital data to an external source.

As mentioned above, cable modem technology is in a unique position tomeet the demands of users seeking fast access to information services,the Internet and business applications, and can be used by thoseinterested in cablecommuting (a group of workers working from home orremote sites whose numbers will grow as the cable modem infrastructurebecomes increasingly prevalent). Not surprisingly, with the growinginterest in receiving data over cable network systems, there has been anincreased focus on performance, reliability, and improved maintenance ofsuch systems. In sum, cable companies are in the midst of a transitionfrom their traditional core business of entertainment video programmingto a position as a full service provider of video, voice and datatelecommunication services. Among the elements that have made thistransition possible are technologies such as the cable modem.

A problem common to all upstream data transmission on cable systems,i.e. transmissions from the cable modem in the home back to the headend, is ingress noise at the head end which lowers the signal-to-noiseratio, also referred to as carrier-to-noise ratio. Ingress noise canresult from numerous internal and external sources. Sources of noiseinternal to the cable system may include cable television networkequipment, subscriber terminals (televisions, VCRs, cable modems, etc.),intermodular signals resulting from corroded cable termini, and coreconnections. Significant sources of noise external to the cable systeminclude home appliances, welding machines, automobile ignition systems,and radio broadcast, e.g. citizen band and ham radio transmissions. Allof these ingress noise sources enter the cable system through defects inthe coaxial cable line, which acts essentially as a long antenna.Ultimately, when cable systems are entirely optical fiber, ingress noisewill be a far less significant problem. However, until that time,ingress noise is and will continue to be a problem with upstreamtransmissions. The portion of bandwidth reserved for upstream signals isnormally in the 5 to 42 MHz range. Some of this frequency band may beallocated for set-top boxes, pay-per-view, and other services providedover the cable system. Thus, a cable modem may only be entitled to somefraction (i.e., a "sub-band") such as 1.6 MHz, within a frequency rangeof frequencies referred to as its "allotted band slice" of the entireupstream frequency band (5 to 42 MHz). This portion of thespectrum--from 5 to 42 MHz--is particularly subject to ingress noise andother types of interference. Thus, cable systems offering two-way dataservices must be designed to operate given these conditions.

Although not fully agreed to by all parties in the cable TV and cablemodem industry, an emerging standard establishing the protocol fortwo-way communication of digital data on cable systems has been definedby a consortium of industry groups. The protocol, known as theMultimedia Cable Network System (MCNS), specifies particular standardsregarding the transmission of data over cable systems. With regard tothe sub-band mentioned above, MCNS specifies that the bandwidth of adata carrier should generally be 200 KHz to 3.2 MHz. Further referencesto MCNS standards will be made in the specification.

As noted above, ingress noise, typically narrow band, e.g., less than100 KHz, is a general noise pattern found in cable systems. Upstreamchannel noise resulting from ingress noise adversely impacts upstreamdata transmission by reducing data throughput and interrupting service,thereby adversely affecting performance and efficient maintenance. Onestrategy to deal with cable modem ingress noise is to position themodem's upstream data carrier in an ingress noise gap where ingressnoise is determined to be low, such as between radio transmission bands.The goal is to position data carriers to avoid already allocated areas.

Ingress noise varies with time, but tends to accumulate over the systemand gathers at the head end. In addition, while a particular frequencyband may have been appropriate for upstream transmissions at thebeginning of a transmission, it may later be unacceptably noisy forcarrying a signal. Therefore, a cable system must attempt to identifynoisy frequency bands and locate optimal or better bands for upstreamtransmission of data at a given time.

Block 104 of FIG. 1 represents a cable modem termination systemconnected to a fiber node 108 by pairs of optical fibers 106. Theprimary functions of the CMTS are (1) receiving signals from externalsources 100 and converting the format of those signals, e.g., microwavesignals to electrical signals suitable for transmission over the cablesystem; (2) providing appropriate MAC level packet headers (as specifiedby the MCNS standard discussed below) for data received by the cablesystem, (3) modulating and demodulating the data to and from the cablesystem, and (4) converting the electrical signal in the CMTS to anoptical signal for transmission over the optical lines to the fibernodes.

FIG. 2 is a block diagram showing the basic components of a cable modemtermination system (item 104 of FIG. 1). Data Network Interface 202 isan interface component between an external data source and the cablesystem. External data sources (item 100 of FIG. 1) transmit data to datanetwork interface 202 via optical fiber, microwave link, satellite link,or through various other media. A Media Access Control Block (MAC Block)204 receives data packets from a Data Network Interface 202. Its primarypurpose is to encapsulate a MAC header according to the MCNS standardcontaining an address of a cable modem to the data packets. MAC Block204 contains the necessary logic to encapsulate data with theappropriate MAC addresses of the cable modems on the system. Each cablemodem on the system has its own MAC address. Whenever a new cable modemis installed, its address must be registered with MAC Block 204. The MACaddress is necessary to distinguish data from the cable modems since allthe modems share a common upstream path, and so that the system knowswhere to send data. Thus, data packets, regardless of format, must bemapped to a particular MAC address.

MAC Block 204 also provides ranging information addressed to each cablemodem on its system. The ranging information can be either timinginformation or power information. MAC Block 204 transmits data via aone-way communication medium to a Downstream Modulator and Transmitter206. Downstream modulator and transmitter 206 takes the packet structureand puts it on the downstream carrier. It translates the bits in thepacket structure to 64 QAM in the downstream (and 16 QAM or quadraturephase shift keying (QPSK) is used on the upstream path). Thesemodulation methods are known in the art and are also specified in theMCNS protocol. It should be noted that optical fibers transmit data inone direction and coaxial cables can transmit data in two directions.Thus, there is only one coaxial cable leaving the fiber node which isused to send and receive data, whereas there are two optical fiber linesfrom the fiber node to the downstream and upstream modulators.

Downstream Modulator and Transmitter 206 converts the digital datapackets to modulated downstream RF frames, such as MPEG or ATM frames,using quadrature amplitude modulation, e.g. 64 QAM, forward errorcorrecting (FEC) code, and packet interleaving. Converter 208 convertsthe modulated RF electrical signals to optical signals that can bereceived and transmitted by a Fiber Node 210. Each Fiber Node 210 cangenerally service about 500 subscribers. Converter 212 converts opticalsignals transmitted by Fiber Node 210 to electrical signals that can beprocessed by an Upstream Demodulator and Receiver 214. This componentdemodulates the upstream RF signal (in the 5-42 Mhz range) using, forexample, 16 QAM or QPSK. It then sends the digital data to MAC 204.

A prior art method of locating an area of lower noise in an upstreampath involves arbitrarily selecting frequencies from a frequency list assoon as the noise for a current frequency becomes unacceptable. Thefrequencies may be chosen using a round robin or other selectionmethodology. Another method involves deploying a spectrum analyzer tolocate an appropriate frequency in a single pass. The first blind "roundrobin" method of picking a frequency from a frequency list (alsoreferred to as dynamic frequency agility) is slow in locating an ingressnoise gap since it requires going through many frequencies before afrequency with an acceptable noise level is located. It also involveschanging upstream data carrier frequencies without measuring orcomparing error levels of the different frequencies before choosing aparticular frequency. Implementing the other method of using a spectrumanalyzer is costly. It involves measuring power levels in the entirefrequency spectrum using a single sweep and identifying ingress noisegaps as power minimas at the head end. Another method utilizes a "gate"that keeps the return path from an individual subscriber closed exceptfor those times when the subscriber actually sends a return signalupstream. This would require knowing when the subscriber will send areturn signal or any signal upstream.

Therefore, what is needed is a reliable, efficient, and cost-effectivemethod of locating upstream ingress noise gaps, thereby enablingdeliberate and intelligent placement of an upstream data carrier in acable network System utilizing cable modems.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurpose of the present invention, a system and method for locatingingress noise gaps for an upstream data carrier in a cable networkutilizing cable modems. In one preferred embodiment, a system foridentifying a transmission frequency that has less noise than otheravailable frequency bands includes a packet generator, a ratecontroller, an echo device, a demodulator, and a packet checker. Theecho device receives test data packets sent downstream by the packetgenerator and rate controller and redirects it upstream at a selectedtest frequency. The packet checker then receives and analyzes the testpacket sent from the echo device to assess the amount of noise on thatparticular test frequency. In a preferred embodiment, the echo device isa subscriber cable data modem configured to receive a test packet sentdownstream and redirecting it upstream at a selected test frequency.

In another preferred embodiment, the echo device includes a media accesscontrol address, a downstream receiver and demodulator, an echo logicdevice, and an upstream modulating and transmitting mechanism. The echologic device replaces the echo device's media access control addresswith the address of the packet checker thereby redirecting the otherwiseunmodified test data packet to the packet checker.

In another aspect of the invention, a method of locating a transmissionfrequency that has relatively less noise than other frequencies fortransmitting digital data upstream in a cable system is described. Testdata is transmitted downstream at a downstream frequency and addressedto an echo device located downstream. The test data is received by theecho device and sent back to a test data checker at a test frequency. Anoise level associated with the test frequency is determined byevaluating the test data once it is received by the test data checker.

In another preferred embodiment, the noise level is determined byexamining the number of errors in the test data and calculating thesignal-to-noise ratio for the test frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further aspects, features and advantagesthereof will be more clearly understood from the following descriptionconsidered in connection with the accompanying drawings in which likeelements bear the same reference numerals throughout the variousFigures.

FIG. 1 is a block diagram of a prior art two-way hybrid fiber-coaxial(HFC) cable system utilizing a cable modem data transmission.

FIG. 2 is a block diagram showing the prior art basic components of acable modem termination system.

FIG. 3 is a block diagram showing a cable modem termination systemaltered for conducting asymmetric echo loop testing of the describedembodiment of the present invention.

FIG. 4 is a block diagram showing a CMTS and an echo device locatedoutside the CMTS.

FIG. 5 shows the internal functional components of an echo device usedin the described embodiment.

FIGS. 6A and 6B are flowcharts showing a method of locating ingressnoise gaps on an upstream path as stated in the described embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Important issues regarding the expansion of the existing cable systeminfrastructure to include full service two-way communication of digitaldata are reliability, low maintenance, and data integrity. The servicemust have low maintenance costs and must not alter data unintentionallyduring transmission. A high rate of data integrity is extremelyimportant to most users. One way data is affected during transmission isby ingress noise, particularly on the upstream data path. The presentmethods for detecting unacceptable noise levels and locating anacceptable noise level are too slow or too costly. The present inventionaddresses the issue of data integrity on locating ingress noise gaps onthe upstream data path where the upstream RF signal can be placed sothat the carrier-to-noise ratio is optimized. It may accomplish this byadding components to a conventional CMTS (described with respect to FIG.2) and placing a custom echo device either internally or externally ofthe CMTS. A preferred embodiment of the alterations to the CMTS is shownin FIG. 3.

FIG. 3 is a block diagram showing a cable modem termination systemaltered for conducting asymmetric echo loop testing. In addition to thecomponents described with respect to FIG. 2, i.e., the data networkinterface, the MAC Block, downstream modulator and upstream demodulator,FIG. 3 includes additional components used for echo loop testing in thedescribed embodiment. Specifically, it includes a modified MAC (MediaAccess Control) Block 302 containing two new components: a packetgenerator 304 and a packet checker 306.

Packet generator 304 creates data packets addressed to a specializedcable modem referred to as an echo device 308 (described in furtherdetail with respect to FIG. 5). The test data packet size and payloadmay be varied depending on what type of test is being conducted, e.g., atest using an ICMP (Internet Control Message Protocol) test data packetor a test using a DFT (Design For Test Protocol) test data packet (bothusing 32-bit data packets). The packet generator essentially takes a bitstream which is used in telephony testing and breaks up the bitstream tocreate the test data packets. It then assigns to each data packet a MACaddress for echo device 308. The bit stream may be a quasi-randomsequence of bits. In another preferred embodiment, IP ping packets canbe used to create test data packets. The type of data packet depends onwhat kind of test is being performed. The bit patterns of the packetsgenerated by the packet generator may be chosen to be a pattern that ishighly susceptible to noise. However, this may be difficult given thatin standard media access control circuitry the MAC sometimes scramblesdata in the packets to promote proper encoding of the signal in QAMformat. In addition, the MAC may interleave bits in the data packet.

A rate controller 310 reduces the data packet rate on the downstreampath so that it matches that of the upstream path. Rate controller 310is necessary in the described embodiment due to the asymmetric nature ofthe system. Echo loop testing requires some form of rate control in thedownstream to avoid overflowing the upstream path in the describedembodiment. The frequency of the downstream path is in the 50 to 860 MHzrange and transmits data at a rate upto 30 Mbps while the frequency ofthe upstream path is in the 5 to 42 Mhz range and carries data closer to1.3 Mbps. In the described embodiment, rate controller 310 provides thenecessary control by filling excess bandwidth on the upstream path withnon-data bits or packets not associated with the loop test. This isnecessary to prevent the inevitable loss of test data packets that wouldoccur if the upstream carrier could not transmit all the data packetsbeing sent on the downstream path. In other preferred embodiments, ratecontroller 310 can simply send groups of IP ping packets and wait fortheir return before sending more packets. In yet other preferredembodiments, it can involve more sophisticated software and hardwaretechniques for throttling the downstream packet rate. While ratecontroller 310 is shown as a separate entity within the CMTS, it may beprovided at other locations. For example, in another preferredembodiment it may be located within

Echo device 308 may be a custom-built device or simply a normal cablemodem. In either case, echo device 308 is registered with MAC block 302and has its own MAC address. It listens on the downstream path for datapackets with its address. In the described embodiment echo device 308 islocated in the CMTS as shown in FIG. 3. Thus, the device is contained inthe CMTS's physical enclosure and is implemented as part of thehardware. It should be noted that packet generator 304, packet checker306, and rate controller 310 can be implemented as software on existinghardware components of the CMTS (e.g. MAC block 302 ). However, echodevice 308 should be implemented in hardware in the described embodimentbecause it needs to provide its own MAC address at a location downstreamfrom packet generator 304 and rate controller 310.

FIG. 4 is a block diagram showing an echo device 404 located outside aCMTS 402. In this preferred embodiment, echo device 404 is implementedas a hardware device installed on the cable system. External echo device404 can be located at any point on a downstream path 406. In anotherpreferred embodiment, the echo device can receive data in radiofrequencies or intermediate frequencies which are more manageable. Anamplifier (not shown) converts the intermediate frequencies to radiofrequencies at a point downstream from the echo device. Intermediatefrequency output is preferable because it allows the elimination of RFcircuitry from a noisy digital board.

In yet another preferred embodiment, echo device 404 is simply a cablemodem installed at a subscriber's premises. In this embodiment, thedevice would most likely be dedicated to echo loop testing so that itdoes not interfere with the user's normal two-way digitalcommunications. The dedicated echo device/cable modem will, as usual,have its own MAC address so that it receives only test data packets fromthe packet generator and, after changing the source and destination MACaddresses will send the same data packet back to the packet checker. Theecho device essentially listens for downstream packets addressed to itand repackages those packets for transmission upstream to packet checker306. As always, the packet generator generates test packets for the looptest that are addressed to the echo device. In preferred embodimentswhere the cable modem is external of the CMTS, the cable modem need notbe physically installed at a subscriber's premises, but may be placedanywhere downstream from the CMTS on the cable system.

Regardless of the echo device's location, test data packets are receivedand analyzed by packet checker 306. It verifies that the data packetsreceived match the data packets transmitted. Packet checker 306accumulates error statistics such as bit error rates (BER), FEC blockstatistics, and packet error statistics. Once the echo loop test isestablished, the upstream carrier frequency is periodically changed atthe upstream receiver and demodulator 312 and correspondingly at theecho device so that error statistics may be gathered at packet checker306 for different frequencies between 5 and 42 MHz. The downstreamcarrier may remain at a fixed frequency during this particular process.Error statistics are sampled for a finite length of time, ΔT, whichindicates the time between changes in frequency (this defined timeperiod is discussed with respect to FIG. 6B). These error statistics areaccumulated for each upstream carrier band within sampling period ΔT. Aningress noise gap is identified when data loss is below a specifiedmaximum limit.

FIG. 5 shows the primary internal functional components of an echodevice used in a preferred embodiment. The device receives radiofrequencies in the range of 50 to 860 MHz that first enter a downstreamreceiver and demodulator 502. Regardless of where the echo device islocated, it listens for and intercepts data packets that have its MACaddress. The next functional component in an echo device is the mediaaccess control and echo logic block 504. This component assigns the testdata packets new addresses corresponding to the CMTS MAC block 302 sothat the data packets will be sent to the packet checker 306. Echo logicblock 504 does not alter the data contents of the data packets butsimply repackages them with new headers specifying the new source anddestination information. Upstream modulator and transmitter 506 thenmodulates the radio frequency to the range of 5 to 42 MHz and transmitsthe test data packets to the packet checker. In a preferred embodimentreceiver 502 and transmitter 506 are simply comparable componentsemployed in a conventional cable modem.

As mentioned, the echo device as shown in FIG. 3 can be either internalto the CMTS or can be located outside the CMTS on the cable network asshown generally in FIG. 4. When the echo device is internal to the CMTS,it is essentially transparent and is implemented as part of the CMTShardware (although it always has its own MAC address). While an echodevice test set up as described above captures essential components ofsuch a system, it should be understood that those components may besituated within a considerably more complex conventional cable system.In most installed cable systems, a splitter splits the downstreamcontent, distributing it to several fiber nodes. One of the streamscontains the echo device. Before entering the echo device, thedownstream path goes through a high pass filter. The other streams fromthe splitter are directed to other fiber nodes and are sent downstream.The echo device is on the wire listening for packets addressed to it. Asexplained, the echo device essentially echoes the packets back to thepacket checker in the CMTS at a lower frequency, thus, the need for thehigh pass filter. Another component in the echo device test set is anattenuator which acts as a cable and essentially prevents overload ofthe upstream channel. Also, in the echo device test set up, thedownstream frequency is set in the upconverter and the upstreamfrequency is set in the echo device. Both are programmed for the samefrequency.

FIGS. 6A and 6B are flowcharts showing a method of locating ingressnoise gaps on an upstream path as stated in the described embodiment.The described method can run at all times (continuous operation) or canbe executed only upon suspicion that there is too much noise in thecable system (discrete operation). Further, the flowchart describesessentially an operational flow assuming that the devices, such as theCMTS and the echo device, are in working condition. The primary purposeof the method described is to find a frequency band slice with the leastamount of noise, i.e., an ingress noise gap.

In a step 600 the system checks to see whether there is too much noisein the cable system. If the noise level is acceptable, the system is ina wait state and continues to check the noise level. If there is toomuch noise in the system, i.e. the signal-to-noise ratio is below anacceptable amount, the system proceeds to a step 602 where a new testsub-band is selected. In the described embodiment the frequency range ofthe sub band which is determined in step 602 is within 5 to 42 MHz, thefrequency range of the upstream path. The sub-band itself can be in anyrange but is typically small, e.g. 100 KHz to 3.2 MHz. Because the fullrange of upstream frequencies is a shared system, the frequency range ofthe cable modem system would be allocated a certain range within 5 to 42MHz. For example, the range above 35 MHz might be allocated to set topboxes. In such cases, the sub-band selected in step 602 will besomewhere within the range of 5 to 35 MHz.

In step a 604 the system sets the cable modem transmit frequency and theCMTS receive frequency to be the same. The power level is also initiallyset at step 604, as is a power level decrement value. Note that thepower level is initially set to a high level and then decremented (atthe selected frequency) to determine the carrier-to-noise ratio. Thepower is decremented by the set amount from a high to low level for eachnew frequency. After the carrier-to-noise ratio for a given frequency isidentified, a new frequency must be selected and subjected to the sameanalysis. The new frequency is set according to the, a frequencyincrement as shown at step 604.

In a step 606 the system sets the frequency of the upstream carrier tothe initial frequency. In a step 608 the system activates andinitializes the packet generator and packet checke. At this point thepacket generator begins sending test data packets. In step 610 thepacket generator begins sending test data packets addressed to the echodevice for a defined time period. Then at a step 612, the packet checkerreceives "echoed" test packets for a period of time equal to ΔT. One wayto derive this time period is to take the sum of the time needed toaccumulate all the test packets that were sent by the packet generatorand received by the packet checker, plus the time delay for the lastpacket as indicated in a step 612. The time delay for the last packet isthe time it takes for the packet to leave the packet generator and reachthe packet checker.

In a step 614 of FIG. 6B, the packet checker conducts an error analysisusing the number of lost packets never received and the number of"errored" packets received and the total number of packets sent. Itsaves the error statistics in an array or database for the frequencybeing analyzed. A final analysis is done when all the frequencies havebeen checked as discussed in greater detail with respect to step a 628.The system stores the lost and errored data packets in a step 616.

In a step 618 of FIG. 6B, the system checks to see whether the lowestpower level has been reached. If it has not been reached, in a step 620the power level is decremented by the amount set in step 604 and thesystem returns to step 608 where the system reinitializes the packetgenerator system then repeats new power level. The system then repeatssteps 610 to 618 as described. If the lowest power level has beenreached the system calculates the signal-to-noise ratio for the presentfrequency in a step 622. The system then checks whether anotherfrequency can be checked in a step 624. If the last frequency has beenreached, the system analyzes all the data to find an ingress noise gapby identifying the frequency with the highest signal-to-noise in a step626. At this point the process is complete.

Returning to step 624, if the last frequency has not been reached,control transfers to a step 628 where the system increments thefrequency to a new current frequency. The associated hardware, such asthe echo device transmitter and the CMTS receiver, must be set toreflect the new frequency in order to process it. In a step 630, thesystem resets to the initial power level as set in step 604. The processthen continues with steps following step 608 until step 618 as describedwhere the system checks to see whether the lowest power level for thenew frequency has been reached.

Thus, the flowcharts describe the process wherein error statistics foreach power level for each frequency is checked before a determination ismade as to which frequency has the best or the highest signal-to-noiseratio. That is, for each frequency all power levels are checkedbeginning typically with the highest power level down to the lowest.After all frequencies have been checked, the system determines which hasthe best signal-to-noise ratio, taking into consideration the powerlevel.

In sum, HFC cable system, the coaxial cable portion of the system is theportion that receives the most ingress noise. As mentioned above,ingress noise can come from various sources ranging from electricalarching from machinery to various types of radio transmissions. Thepresent invention addresses the issue of avoiding ingress noise on acable modem upstream carrier by locating ingress noise gaps andpositioning an upstream data carrier in one of the gaps.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Furthermore, although only a few embodiments of thepresent invention have been described, it should be understood that thepresent invention may be embodied in many other specific forms withoutdeparting from the spirit or the scope of the present invention. Forexample, the echo device can be located within the CMTS or anywhere onthe downstream path including a subscriber's home. In another example,the echo device itself can be a cable modem with a special MAC addressor a custom-built device located on the cable system.

What we claim is:
 1. A method of testing the reliability of an upstreamdata path in a cable system using a cable modem device configured foruse in a variable carrier frequency loop testing system, the methodcomprising:receiving a downstream test packet at a demodulator in acable modem device, the test packet having a first media access controladdress of the cable modem device; replacing the first media accesscontrol address with a second media access control address correspondingto a test packet checker device; and transmitting the test packetupstream to the test packet checker.
 2. A method as recited in claim 1further comprising transmitting a downstream test packet at a downstreamfrequency, the test packet being addressed to the cable modem device. 3.A method as recited in claim 1 further comprising determining a noiselevel associated with the upstream data path by evaluating the receivedtest packet.
 4. A method as recited in claim 3 furthercomprising:determining the number of errors in the test packet byanalyzing the test packet received by a test data receiver, the testdata receiver being part of a destination node; storing in memory thenumber of errors in the test packet; and calculating the signal-to-noiseratio for the upstream data path.
 5. A method as recited in claim 1wherein a pre-determined number of test packets are sent to the testpacket checker device such as a fixed number of data packets or a fixednumber of bits.
 6. A method as recited in claim 1 further comprisingcontrolling the rate at which a test packet is sent downstream whereby aplurality of test packets is not sent downstream at a rate faster thanan upstream data transmission rate.
 7. A method as recited in claim 1further comprising partitioning a quasi-random bit stream to create aplurality of test packets.
 8. A method as recited in claim 1 furthercomprising accessing the signal-to-noise ratio of the upstream data pathby accumulating statistics including a bit error rate, forward errorcollection block statistics, and packet error statistics.
 9. A method asrecited in claim 1 further comprising configuring the cable modem deviceto receive the test packet sent downstream and redirecting it upstreamat a selected test frequency.
 10. A system for identifying atransmission frequency for transmitting digital data upstream in a cablesystem, said transmission frequency having relatively less noise than atleast some other frequency bands, the system comprising:a packetgenerator for generating one or more test packets at a selectedfrequency and transmitting a test packet to an echo device locateddownstream on the cable system; a packet checker for receiving the testpackets sent by the echo device at the selected frequency, wherein thepacket checker assists the system in determining the transmissionfrequency for transmitting digital data upstream by evaluating the testpackets to determine a noise level.
 11. A system as recited in claim 10further comprising a test packet analyzer in a destination nodeincluding a memory area for analyzing the number of errors in the testpacket.
 12. A system as recited in claim 10 further comprising:a ratecontroller for controlling a test packet generation rate of the packetgenerator such that a downstream packet rate does not exceed the packetrate capacity of the upstream path.
 13. A system as recited in claim 12wherein the rate controller is contained in a media access controldevice.
 14. A system as recited in claim 10 wherein the echo device iscapable of receiving the test packet sent downstream and redirecting itupstream at the selected frequency.
 15. A system as recited in claim 10further comprising a demodulator which can receive the test packet sentupstream at the selected frequency.
 16. A system as recited in claim 15wherein the packet generator and the demodulator are capable oftransmitting the test packet at a plurality of test frequencies.
 17. Asystem as recited in claim 10 wherein the packet generator and thepacket checker are contained in a media access control device.
 18. Asystem as recited in claim 10 wherein the packet generator is capable ofpartitioning a quasi-random bit stream to create the one or more testpackets.
 19. A system as recited in claim 10 wherein the packet checkerfurther includes a error statistic accumulator for accessing thesignal-to-noise ratio of the selected frequency by accumulatingstatistics including a bit error rate, forward error collection blockstatistics, and packet error statistics.
 20. A system as recited inclaim 10 wherein the echo device is a subscriber cable modem having aunique media access control address.
 21. A system as recited in claim 10wherein the echo device further includes;a media access control address;a downstream receiver and demodulator device capable of intercepting atest packet addressed to the echo device and of forwarding the testpacket; an echo logic device for replacing the media access controladdress of the echo device contained in the test packet with a mediaaccess control address of the packet checker; and an upstream modulatingand transmitting mechanism for transmitting the test packet.